The present invention relates to a process for the production of a resistive layer for an acoustic wall, particularly for the production of nacelles of aircraft jet engines and more generally any conduit requiring soundproof walls.
The invention also covers the resistive layer obtained and an acoustically absorbing wall using this layer in combination with other layers.
There are known resistive layers that are more or less permeable to air which permit very significantly attenuating sound waves. These layers are combined with open work structures of the honeycomb type to constitute quarter wave resonators connected to a total reflector.
Resistive layers have the role of dissipating acoustic energy by transforming it into heat thanks to the viscous effects engendered during flow of the waves. They generally comprise at least one acoustically absorbent cloth and a reinforcing material.
Such layers as well as panels formed from these layers are disclosed in French patent application No. 2 767 411 in the name of the present applicant. In this application, it is provided to reinforce the mechanical resistance of a metallic cloth or sound deadening composite, by addition of a layer of structurally reinforcing material, connected to this resistive layer. In this application, reinforcing filaments are of adjustable open surface and secured to said cloth.
The acoustic cloth is essentially selected as a function of the high capacity to perform linear acoustic processing and to trap the acoustic waves in Helmholtz cells formed by the open work structure. This cloth is of a suitable mesh but its thickness is necessarily very low, of the order of 1 to 2 tens of a millimeter to give an order of size.
In the case of the choice of a metallic sound deadening cloth, there can be used a stainless grill of the type of those sold under the mark GANTOIS.
Such fabrics have the advantage of being available on the market and even with very low thicknesses as indicated, the mechanical resistance remains great relative to a fabric of synthetic material.
Thus, in the case of the nacelles of aircraft jet engines, the surface of the resistive layer is immediately in contact with solid particles such as grains of sand and small stones which give rise to erosion phenomena or else pieces of ice or possibly birds that are sucked in, which, at that speed, give rise to mechanical degradation.
The metallic fabric also has the advantage of conducting lightning well.
A first drawback remains its weight relative to synthetic materials, which also explains the very reduced thickness so as to limit excessive weight.
Another important drawback is the connection between this fabric and the reinforcing material, which is a perforated plate of light metal such as aluminum, a molded composite panel that is shaped or of filaments (which is to say strips of filaments, roving, braids or strands of filaments, according to the cross-section).
This connection is very important because in the case in which the fabric is disposed on the outside, on the side of the circulating air flow, it is necessary to avoid any delamination of the fabric relative to its support, particularly in the case of starting a mechanical rupture engendered accidentally or by a foreign body.
Thus, if delamination takes place, the pieces of fabric of greatest surface can tear off, which would be unacceptable.
Moreover, another problem is that of connecting the fabric to its support whilst maintaining the pores because any decrease in the quantity of holes (quantity of open surface) contributes to decreasing the sound deadening capacity of the resistive layer.
In the case in which the acoustic deadening cloth is interposed between the reinforcing layer and the honeycomb structure as provided in the specific arrangement according to the related patent application in the name of the same applicant, the connection problem is also very great. In this case, it is necessary that the connection between the open work honeycomb structure and the reinforcing layer takes place on opposite sides of the acoustic deadening fabric or even in part through the pores but always without closing these pores.
The techniques used in the prior art consist in resorting to thermosetting resins, but the control of this family of resins is difficult, and moreover, the connection gives lower resistance than the intrinsic resistance of the fabric itself, which it to say the filaments which comprise it, such that the bonding zone remains the weakest point of the resistive layer in its assembly.
The object of the present invention is to provide a process for the production of a resistive layer to produce a connection of its constituents, in different forms of arrangement, which is satisfactory by having capacities of solidarity such that, mechanically for example, the resistance of the connection at the interface of the structural and acoustic components is greater than the intrinsic resistance of the acoustic deadening fabric, thereby forming a monolithic assembly.
The invention also covers the resistive layer obtained and walls provided with such a resistive layer.
To this end, the process of producing an acoustically resistive layer according to the present invention comprises the following steps:
producing a structural component by using thermoplastic resins, this component having a given open surface quantity related to the acoustic waves to be treated,
connecting a metallic acoustic cloth whose mesh is suitable for the open surface quantity of the structural component, and
carrying out the polymerization of the thermoplastic resins under pressure and at high temperature.
The family of resins preferably used comprises:
PEI: polyetherimides,
PEEK: polyetheretherketones,
PPS: polyphenelenesulfides,
PA: polyamides, and
PET: polyethyleneterephthalate
According to embodiments, the resistive layer is obtained by shaping on a mold metallic acoustic fabric and by depositing or by winding filaments impregnated with thermoplastic resins forming the structural component on this fabric, or the other way around, and by the step of polymerization of the thermoplastic resins.
There can thus be produced the resistive layer by emplacement on a mold of a sheet of composite material comprising thermoplastic resins and added to this sheet metallic acoustic fabric, or the other way around, these steps being followed by polymerization of the thermoplastic resins.
There are utilized filaments of mineral and/or organic origin.
The invention covers the acoustically resistive layer obtained by the practice of these steps of the process, and the associated acoustically absorbing wall.
The process of production of a resistive layer will now be described in detail in the case of an assembly of practical embodiments, as well as the resistive layers obtained and the acoustically absorbent walls produced by using at least one such resistive layer.